A motor generator which functions selectively as an electric motor and a generator is mounted in hybrid cars or electric vehicles. Such a motor generator comprises, for example, a cylindrical rotor fixed to an output shaft supported rotatably about the shaft center, a motor stator having an inner circumferential surface which is disposed with a predetermined gap to the outer circumferential surface of the rotor, and a motor housing for accommodating the motor stator.
Since the motor generator generates heat by the operation such as acceleration and deceleration of a motor vehicle to be heated to a high temperature of 100° C. or more, it is required to be sufficiently cooled in order to maintain a predetermined performance.
As a method for effectively cooling the motor generator, there has been known a method of supplying a cooling oil into the housing, immersing the motor stator and rotor in the cooling oil, and circulating the cooling oil. (Patent Document 1)